Age related changes in immunologic and biochemical aspects of lymphoid cell function are currently being studied. T lymphocytes from young and aged C57B1 mice are being fractionated in an attempt to compare functional subsets of these cells by immunologic (helper and suppressor activity) and biochemical (5' nucleotidase activity and incorporation of tritiated thymidine into DNA) criteria. One way mixed lymphocyte reactions are also being carried out in an attempt to determine whether T cells from young and aged C57B1 mice show autoreactivity.